Four Forces of Flight

The faster an airplane moves forward, the greater the lift produced. This is why an airplane accelerates on the runway. When speed and lift have increased to sufficient levels, the plane will become airborne. Conversely, a plane coming in for a landing must be slowed to reduce lift. To compensate for the lower air speed, the pilot tilts segments of the wings downward, thereby pushing more air from under the wings and maintaining sufficient lift to prevent the plane from falling out of the sky.

A second force, weight, opposes lift. Weight is the measure of gravity’s effect on the airplane’s mass. Earth’s gravity “pulls” the plane down toward the ground. To overcome the force of gravity and achieve flight, an airplane must generate more lift than the total weight of the plane, its fuel, and all its contents. If an airplane loses its lift, weight (gravity) causes it to come crashing down.

The other two forces of flight are thrust and drag. Thrust is the forward force created by the propellers or jet or rocket engines as they blow air or exhaust backward to propel the plane. Drag—friction with the air while a plane is moving forward—works in the opposite direction. To fly forward, an airplane must produce thrust greater than the forces of drag impeding it.

Aeronautical engineers try to streamline their airplane designs so that planes cut through the air smoothly. A plane with a lot of drag will not be very efficient. As long as lift is greater than weight and thrust is greater than drag, any shape of airplane will fly.

References:

Vogt, G.L., B.Z. Tharp, M.T. Vu, and N.P. Moreno. 2014. Think Like an Engineer Teacher’s Guide. Baylor College of Medicine (ISBN: 978-1-888997-64-4). Development of Think Like an Engineer educational materials was supported, in part, by National Science Foundation grant number DRL-1028771.